Mild steel has a wide array of industrial applications, but the tendency to corrode limits its service life in various engineering applications. Phosphate and chromate based conversion coatings are used for many years for corrosion protection purposes. But, these conversion coatings are discouraged over the past few years because of the strict environmental issues and human health concerns. As an alternative approach, eco-friendly paints and coatings are explored for corrosion protection of active metals. However, designing of such coatings having corrosion protection efficiency equivalent to their traditional counterparts is still a challenge.
Chitosan is a natural polysaccharide found in the shells of marine crustaceans, a common sea food waste. The proper disposal of these sea food wastes is a continuous problem. They create major environmental concern associated with ground and drinking water pollution. Chitosan, a bio waste, can be used as an eco-friendly candidate for coating purposes because of its excellent film forming tendency, low cost, most plentiful abundance in nature, biodegradable and non-toxic nature. The film forming nature of chitosan can be used to design barrier coatings for corrosion resistance of active metals as it forms complexes with metal ions and also adheres to negatively charged surfaces. However, chitosan is prone to absorb moisture leading to the failure of the coating on prolong exposure to hostile environments. A few investigations have been carried out to decrease the hydrophilicity and to improve bond strength of chitosan to its substrate. However, these measures are not sufficient for long term corrosion protection of active metals.
Conjugated polymers are reported as corrosion inhibitors for some active metals and alloys. However, mechanical integrity and thermal stability of these polymer based coatings are low. The interaction of chitosan and conjugated polymers like polyaniline, polypyrrole has been reported earlier. The synergistic effect of chitosan and polypyrrole is reported to reduce the size and improve the solubility of polypyrrole. In view of this, chitosan and polypyrrole system can be explored as corrosion resistant coatings for metals like mild steel. However, the potential of chitosan-polypyrrole composite as a corrosion resistant coating material is not studied well.
Organic/inorganic composites have attracted academic and industrial interest in recent years. These composites are organic polymers with inorganic fillers, which significantly improve the properties of the polymer. Therefore, these composites can be explored as coating material having superior properties. Polyaniline/SiO2 composite coatings have been reported to enhance the corrosion resistance of steel substrates. Incorporation of SiO2 particles as filler in the polymer matrix has shown significant impact on thermal and mechanical properties of the coatings. Silica (SiO2) has a large surface area and smooth non-porous surface. This feature promotes contact between SiO2 (as filler) and the polymer matrix. However, the chemically synthesized polymer composites are difficult to apply as coatings because of their non-solubility in common organic solvents.
Conventional polymer coating like epoxy is known to have superior characteristics as compared to regular paints due to its good scratch resistance and adhesion to metal surface. However, epoxy coating alone is not sufficient to protect the underlying metal and fails due to cathodic disbondment under harsh environmental conditions over prolonged exposure. Literature shows the use of SiO2 particles as reinforcement for anti-corrosive epoxy coatings. The particles moderately improve the corrosion resistance properties of epoxy coatings in aggressive medium. However, very limited work is reported on epoxy paints containing polyaniline and polypyrrole as anticorrosive additives.
Reference may be made to US patent 20080305341 which relates to coating of metallic surfaces with an anti corrosive coating containing conductive polymers in particulate form with a binder. The conductive polymers used were polyphenylene, polyfuran, polyimidazole, polyphenanthrene, polypyrrole, polythiophene and polythiophenylene. The conductive polymers were doped with anti corrosive mobile anions say, ZrF62−, CeO4 4−, MnO4−, MnO4 2−, MoO4 2−, MoO4 4−, VO4 2−, WO4 2−. The conductive polymer coats the metallic surface and is also dried. A second composition containing a binder system is then applied as a dispersion (solution, emulsion and/or suspension) to the precoated metallic surface and is dried and optionally also polymerized with conductive polymers charged with anti-corrosive mobile anions. The binders are polycarbonate-polyurethane, polyester-polyurethane, styrene, styrene-vinyl acetate, vinyl acetate, vinyl ester and/or vinyl ether. However, the drawbacks thereof include that these coatings use toxic mobile anions and the use of polymer first on the surface followed by coating with binders.
Reference may be made to U.S. Pat. No. 6,756,123 which relates to an anticorrosion paint for preventing steel materials from corrosion by using polyaniline as an anticorrosion pigment. The anticorrosion paint has an excellent anticorrosion effect, long persistence of the anticorrosion effect, high durability and superior coating property controlling the anticorrosion effect, without using metal causing environmental pollution as an anticorrosion pigment. The invention provides a coated steel substrate comprising an anticorrosion paint system, wherein the paint system comprise a primer coat paint layer and a top coat paint layer: a primer coat paint layer including 3 to 49 weight percent (wt %) of the polyaniline, 40 to 86 wt % of the matrix resin, 1 to 47 wt % of the additives and 10 to 56 wt % of the mixed solvent; and a top coat paint layer for protecting for protecting the primer coat paint layer and improving the anticorrosion effect, the top coat paint layer including 40 to 89 wt % of matrix resin, 5 to 54 wt % of colored pigment, 1 to 50 wt % of additives and 5 to 54 wt % of mixed solvent. Epoxy resin or acrylic urethane resin was employed as the resin. Titanium oxide, iron oxide, phthalocyanine blue or phthalocyanine green, was used as inorganic or organic pigments. Xylene, toluene, methyl isobutyl ketone, butyl acetate, ethyl acetate, cellosolve acetate, ethyl cellosolve, n-butanol, isobutanol, isopropylalcohol, methyl ethyl ketone, cyclohexanone, methanol or diacetone alcohol were used as solvents. The said anticorrosion paint plays a role in exerting persistently the excellent anticorrosion effect without causing environmental problem. However, the drawbacks thereof include a loading of higher concentration of polyaniline with higher concentration of dopant is a limiting factor for its practical application. Moreover these coatings does not show self-healing characteristic.
Reference may be made to CN 102702920 which discloses a water dispersible polyaniline anti-corrosive coating and preparation method thereof. The invention claims a water dispersible polyaniline anti-corrosive coating which is a green environmental friendly anticorrosion coating and has excellent antiseptic property. The paint comprises a component A and a component B, wherein the component A comprises of epoxy resin, aqueous film-forming co solvent, deionized water, polyaniline nano fiber powder, filler, non-ionic surfactant, defoaming agent, aqueous dispersant; and the component B comprises aqueous curing agent, deionized water and leveling agent. The aqueous polyaniline anti-corrosive paint is a green and environment-friendly type anti-corrosive paint and has an excellent anti-corrosive property. However, the drawbacks thereof include the polymer polyaniline which is showing anti corrosive property in the above invention but does not show self-healing characteristic.
Reference may be made to CN 102101962 which recites a polyaniline epoxy anticorrosive dope and its preparation method. The coating comprises component a, Epoxy resin, mixed solvent, polyaniline nanometer composite material and Component b is comprised of amine firming agent, ethanol without water. The toluene and xylene the invention claims a n-butyl alcohol isobutyl alcohol and tertiary butyl alcohol the invention claims a mixed into mixing solvent the component a and b according to weight ratio of 1:0.5-1.2 mixing and stirring 0.5-1 h can. Whole coating material system the mechanical property of the aging resistant performance corrosion resistant performance is improved obviously the metal surface passivation form a layer of compact stable oxidation film it prevents the metal is further oxidized However, the drawbacks thereof include that the polymer does not show self-healing characteristic.
Reference may be made to CN 102352179 which relates to a preparation method of a polypyrrole anticorrosion coating on the metal surface. The method is characterized in that: a monomolecular film of a pyrrole derivative with two functional groups is self-assembled on the metal material surface, one functional group pyrrole of the monomolecular film is polymerized with pyrrole, and an obtained polypyrrole film which is chemisorbed on the metal material surface is not easy to peel and has an anticorrosion effect. The method, which allows the film which is compacter than common synthesized polypyrrole films to be generated on the metal material surface, allows the penetration of corrosion ions to the coating to be effectively obstructed in a long term, the metal corrosion potential to be improved and the corrosion current to be reduced; and the preparative coating which has the advantages of high adhesion, uneasy peeling, and uneasy scratch because of compactness allows the metal material corrosion to be avoided. However, the drawbacks thereof include the coating to be anticorrosive but the coating does not have the formulation as detailed in our patent application.
Reference may be made to CN 101864239 B which claims a hyper-branched epoxy resin and the polypyrrole composite anticorrosive paint and preparation method thereof. The hyperbranched epoxy resin/polypyrrole composite anticorrosive paint is prepared by simply mixing the component A and the component B according to the proportion and curing at 80 DEG C when in use. The anticorrosive paint prepared in the invention has excellent anticorrosive performance as well as physical and mechanical properties and is environment-friendly. However, the drawbacks thereof include that the patent does not report the life of the coating under severe conditions.
Reference may be made to EP 1723270 which recites a method for protecting a metal surface from galling and corrosion, which includes a step of providing a protective dry film on the metal surface. The film includes a solid lubricant and a conducting polymer, the conducting polymer having lubricant properties and being capable of binding the solid lubricant to the metal surface. Threaded metal joint surfaces coated with the film are capable of resisting galling under high pressure and high torque conditions, even after several fastening and unfastening operations or over a long period of time. Protection from corrosion is also provided by the film. The method and film are economical in that only a single layer of protective compound need be applied in order to provide metal surfaces with both lubrication and protection against corrosion, and problems such as removal or leakage, which are associated with protective compounds that use oils, are avoided. Additionally, the dry film is advantageous because it does not contain heavy metals that are harmful to the environment. However, the drawbacks thereof include that the patent does not report how the coating of conducting polymer with lubricant will behave in hazardous toxic conditions and does not show any smart action
Reference may be made to EP 1779392 which discloses a process for coating fine particles on surfaces, in which the feed mixture contains: at least one monomer and/or at least one oligomer selected from monomers and/or oligomers of aromatic compounds and/or unsaturated hydrocarbon compounds suitable for forming an electroconductive oligomer, polymer, copolymer, block copolymer or graft copolymer; at least one type of anions which (1) are and/or can be incorporated as doping ions into the structure of the conductive polymer; (2) can be discharged from said structure in the event of a potential fall of the conductive polymer (reduction); and (3) can have an anti-corrosive effect in the presence of a metallic surface; at least one type of particles; if necessary, at least one oxidizing agent and water and/or at least another solvent. A coating is formed from the feed mixture on the particle surface, the feed mixture being converted by oxidation into a conductive polymer in the presence of at least one type of mobile anti-corrosive anions. Alternatively, the fine particles are coated with a product mixture that contains a conductive polymer. However, the drawbacks thereof include the patent application does not speak about the suitability of coatings on metal surface in critical conditions and its mechanism.
Reference may be made to U.S. Pat. No. 6,762,238 which describe a coating composition containing a polymeric complex between polyaniline and a polymeric ion. In addition to the said polymeric complex, the composition contains a water-dispersable binder. The composition is useful as a water-borne paint to be applied onto a metal substrate electrophoretically or non-electrophoretically. However, the drawbacks thereof include the use of water borne polymeric complex which may lose its adherence to the metal surface under vigorous salt spray conditions.
Reference may be made to U.S. Pat. No. 8,298,350 which describes preparation of chromium-free conversion coating by the addition of inorganic salts to dispersions of conducting polymers which are then exposed to alloys of aluminum or other metals. The performance of the coating is comparable to that of known Cr (chromium)-based methods for a number of aluminum alloys having particular significance in the manufacture of aircraft. However, the drawbacks thereof include its specific application to Aluminum and its alloys but does not provide smart action under vigrous conditions.
Reference may be made to U.S. Pat. No. 5,993,695 which relates to an aqueous coating dispersion comprising at least electrically conductive particles, which particles comprise at least a first binder, a conductive polymer and a non-ionic stabilizer, characterized in that the coating dispersion contains 50 to 99 wt. %, relative to the total amount of solids present, of a second binder, which second binder does not form part of the electrically conductive particles. The invention also relates to a process for the preparation of the aqueous coating dispersion and use thereof in an anticorrosive paint, to the anticorrosive paint based on the aqueous coating dispersion according to the invention and to the metals protected herewith. However, the drawbacks thereof include higher percentage of polymer binder which will strongly affect corrosion prevention and does not have any self healing mechanism.
Thus, keeping in view the drawbacks of the hitherto reported prior art, the inventors of the present invention realized that there exists a dire need to provide self-healing smart coatings capable of imparting corrosion resistance in marine conditions while being low cost, environment friendly, having excellent finish and which are prepared without any emission of volatile organic compounds (VOCs).